1. Field of the Invention
The present invention is directed to a diagnostic nuclear magnetic resonance (magnetic resonance imaging) apparatus having a main magnetic field and gradient coils for the generation of gradient fields, the gradient coils including conductors that run essentially perpendicularly to the direction of the main magnetic field.
2. Description of the Prior Art
A diagnostic nuclear magnetic resonance device of the above general type is known from U.S. Pat. No. 4,954,781. The nuclear magnetic resonance device disclosed therein has a cylindrical examination chamber that accepts a patient to be examined. The examination chamber is surrounded by a superconducting magnet that generates a homogeneous main magnetic field in the examination chamber extending in an axial direction, i.e. the z-direction. A cylindrical carrier tube is arranged between the superconducting magnet and the examination chamber, to which gradient coils are attached for the generation of gradient fields in directions perpendicular to one another, of which one direction coincides with the direction of the main magnetic field in the z-direction. High-frequency antennas are likewise fastened to the carrier tube, by means of which nuclear spins in an examination subject are excited and the resulting nuclear magnetic resonance signals are received.
In the operation of the nuclear magnetic resonance apparatus for the generation of sectional images, the gradient fields must be switched on and off. This is achieved by supplying the gradient coils with switched currents of different amplitudes and different switching frequencies, with the direction of the currents through the gradient coils additionally being changed. This has the consequence that the conductors of the gradient coils, as well as the carrier tube, are exposed to oscillating forces that produce bothersome noises. In modern imaging sequences, particularly during rapid imaging, these noises reach high levels.
One possibility for the reduction of the noise produced by the gradient coils to arrange noise-damping materials in the vicinity of the gradient coils, which materials absorb the acoustic energy and thereby reduce the production of noise. The noise-reducing effect improves as the mechanical tension or oscillations to which the damping material is exposed becomes higher. The precision of the generated gradient fields, however, is affected by oscillations of the gradient coils. A limit for sound-damping measures is thus that oscillations cannot persist at a level such that the precision of the gradient fields, and thereby the image quality, is affected.
Another measure for the reduction of noises disturbing to the patient is to compensate the noises by also supplying the patient with "compensating noise" formed from the primary noise phase-shifted 180.degree.. This measure, however, has shown only limited success, because the modes of oscillation of the gradient coils are very complex, and for this reason the necessary 180.degree. phase shift cannot be achieved for every mode of oscillation.